1. Field of the Invention
The invention relates to a rolling ball type two-stage low speed changer device in which an eccentric disc is placed between a stationary disc and an output disc having a hypo-based groove or epi-based groove through rollable balls provided therebetween.
2. Description of Prior Art
In general, a rolling ball type speed changer device has been made to work only through a single one reduction stage. Upon determining a speed reduction ratio by e.g., ⅓, the number of lobes of an epicycloidal groove is six, and the number of lobes of a hypocycloidal groove is eight as a combination of the maximum number of lobes of a cycloid-related groove.
Since an entire length of the epicycloidal groove and that of the hypocycloidal groove are theoretically equal respectively, a circumferential difference of unit length appears by the ratio of eight to six per one single lobe.
With the decrease of the speed reduction ratio, the circumferential difference increases. The circumferential difference decreases in proportion with the increase of the speed reduction ratio. The rollable ball moves along the opposed grooves, the mutual length of which differs per one single lobe so as to produce a differential slippage against the groove.
The differential slippage causes an irregular torque-transmission to rotate an output shaft unevenly, thus redering a precise speed reduction movement incapable so as to loose a good transmission efficiency with no small heat production wrought due to a friction between the discs.
Typical rolling ball type speed changer devices have been introduced by Laid-open Japanese Patent Application Nos. 2006-077833 and 2003-172419 (referred to as first and second reference hereinafter).
In the first and second references, a cycloid-based guide groove is formed on both sides of a single eccentric disc. A stationary and output discs are placed to be mutually opposed with the eccentric disc interposed therebetween. To each side of the stationary disc and the output disc, a cycloid-based guide groove is provided.
More specifically, the first reference provides a right and left plate, each of which is mutually opposed and has a cycloid-based annular teeth. Between the right and left plates, a middle plate is provided which has annular teeth in a cycloid-based configuration. Rollable balls are placed between the annular teeth of the right and left plates. In accompany with the rotational movement of an input shaft, the middle plate eccentrically rotates to transmit the torque from the left plate to the right plate through the rollable balls so as to produce a reduced rotation from an output shaft.
On the other hand, the second reference provides first, second and third discs which are juxtaposed to space oppose each other. Each of the discs has a cycloid-based guide groove with rollable balls interposed therebetween. In accompany with the rotational movement of a crankshaft, the second disc eccentrically rotates to produce a reduced rotation from the third disc.
However, in each of the first and second references, there has been no successful example to actually insure a precise and smooth speed reduction. Even though it may be possible to combine any number of lobes between the discs having different cycloid-based grooves, it is by no means easy to implement a precise and smooth rotational transmission.
Why they have been unsuccessful can be generally explained based on the mechanical theory. Namely, there surmisably lies an inconsistent displacement of phase at the trochoidal movement between the two discs observed upon eccentrically rotating the input shaft.
Therefore, the invention is made to obviate the above drawbacks by rendering empirical formulas consistent in phase between lobes of the cycloid-based curves defined on discs to be effective within a practical usage.
It is an object of the invention to a rolling ball type two-stage low speed changer device which is capable of reducing a thickness dimension to render a whole structure compact, achieving a high transmission efficiency without inviting a backlash, attaining a high torque transmission with low noise, and insuring a precice and smooth rotational transmission without inviting the differential slippage and incurring any irregular rotation and uneven torque transmission.